Conventionally, methods for controlling a camera by tracking an object captured in consecutive, time-series image frames and controlling the camera according a result of the tracking have been proposed. For example, Japanese Patent Application Laid-open No. 2008-153879 proposes an automatic tracking image-capturing apparatus that tracks an object in image frames and adjusts a torque value related to camera control by using a motion vector resultant from the tracking.
Japanese Patent Application Laid-open No. 2009-10453 proposes a countermeasure against failure in tracking caused by a tracking error that can occur during object tracking due to an image noise or a change in shape of a tracking-target object. More specifically, a method of continuing tracking without interruption by receiving an input, which is input by a user, of a tracking point for correcting a tracking result has been proposed.
In a general tracking method, object tracking of an object captured in consecutive image frames is performed by determining, in an image frame F0, an initial tracking-target region ABCD as illustrated in FIG. 9 and thereafter performing a tracking process on the object in a subsequent image frame. More specifically, when a tracking-result region A′B′C′D′ in an FM frame (which is the Mth image frame) illustrated in FIG. 10B is obtained in relation to the initial tracking-target region ABCD in the F0 frame (initial image frame) illustrated in FIG. 10A, the tracking process is performed on a subsequent image frame by using the tracking-result region in the FM frame as a new tracking-target region.
However, in a situation where an initial tracking-target region is restricted or the like, when the initial tracking-target region ABCD is selected as illustrated in FIG. 11A, the region A′B′C′D′, which is a region other than an intended tracking-target region, can be undesirably obtained as a result of tracking in the FM frame. In the example illustrated in FIGS. 11A and 11B, only a portion of an object is included in the initial tracking-target region; hence, the initial tracking-target region is inappropriately selected. Accordingly, the tracking-result region undesirably tracks a different portion of the target object, making a tracking result unstable.
A conventional technique, such as that disclosed in Japanese Patent Application Laid-open No. 2008-153879, is disadvantageous in that a probability to fail in the object tracking becomes high if an initial tracking-target region such as that illustrated in FIGS. 11A and 11B is selected.
With the technique of Japanese Patent Application Laid-open No. 2009-10453, in a case where a failure of tracking should occur, correction therefor can be performed; however, if an initial tracking-target region is not selected appropriately, a result of the tracking becomes unstable as well to require frequent manual interventions. This places a large burden on a user.
Due to the above circumstances, an object tracking method of automatically selecting an appropriate initial tracking-target region to reduce a chance of failure in tracking has been desired.
The present invention has been conceived in view of the above circumstances and provides an image capturing apparatus having an object tracking function that yields a stable tracking result from tracking of an object in time-series image frames and achieves an increased tracking success rate, and an image capturing method.